We aim to explore if disease severity score (DSS) at onset is associated with dialysate white cell counts, the severity of causative organisms and the risk for treatment failure of peritoneal dialysis (PD)-associated peritonitis in an adult PD cohort. Our prospective cohort study recorded all peritonitis episodes between 2008 and 2010. The DSS, demographic data and clinical characteristics were recorded at the onset of peritonitis. The dialysate cells were counted at regular intervals and organism culture were examined too. Treatment failure of peritonitis was defined as peritonitis-associated death and transfer to haemodialysis. A total of 219 episodes of peritonitis in 146 PD patients were recorded, 21.9% of which resulted in treatment failure. There were no significant differences in dialysate white cell counts on the fifth and seventh day and the distribution of causative organism between groups with varied DSS level. DSS could not predict treatment failure including peritonitis-related death and transfer to haemodialysis after adjusting for age, gender, diabetes, dialysis duration, dialysate white cell count on the third day, the presence of Staphylococcus aureus, gram-negative organisms and polymicrobial organisms. Our study demonstrated that DSS at onset was not associated with prolonged elevation of dialysate white cell counts, severity of causative organisms and outcome of peritonitis episodes in adult PD patients.